Alloy steel and method



United States Patent 3,123,506 ALLOY STEEL AND METHOD Harry Tanczyn, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio No Drawing. Filed Oct. 6, 1960, Ser. No. 60,803 11 Claims. ((31. 148-61) My invention relates generally to the alloy steels, more particularly, heat-hardenalble nickel-molybdenum alloy steel as well as a method for conditioning such a steel for hardening, and to the products fashioned therefrom, both in the prior-to-hardening condition, and in the heat-hardened form.

One of the objects of my invention is to provide a nickel-molybdenum alloy steel, as well as various fabricated products fashioned thereof which are capable of being hardened from an annealed condition with a singlestep, low-temperature ageing treatment or, as desired, with a double-step treatment, to achieve hardness and strength under sustained operating conditions of both high and low temperatures.

A further object is to provide a method for conditioning alloy steels of the type indicated wherein, following shaping, forming or other fabrication of the metal in annealed condition or in a partially hardened condition, the metal thereafter, and in one-step operation, or with a double treatment as desired, is readily hardened and strengthened by way of a simple and comparatively low-temperature ageing treatment of relatively short duration.

Other objects and advantages of my invention will be obvious in part and in part more fully pointed out during the course of the following description.

In general, it may be said that my invention resides in the combination of component elements and in the composition of ingredients, and in the several operational steps and the relation of each of the same to one or more of the others, all as described herein, the scope of the applioation of which is indicated in the claims at the end of this specification.

In order to better understand certain features of my "invention, it may be noted at this point that the known nickel-molybdenum steels are usually of rather low nickel and low molybdenum contents but with high carbon contents. These steels are hardenable by quenching in oil or water from a temperature of some 1450 to 1525 F. Subsequent to quenching they ordinarily are tempered at about 800 F. to relieve stresses set up in the steel as an incident to hardening.

Typical of the known nickel-molybdenum steels are the SAE 4600 and 4800 series. The 4600 series have a nickel content of about 1.5% to 1.8%, a molybdenum content of about .2% to .25%, with carbon typically about .40% to .45%, and remainder iron. The composition of the 4800 series is similar, the nickel content being about 3.5% the molybdenum about .25 the carbon typically .40% to .45 and remainder iron. These steels are somewhat difficult to weld, requiring pre-heating and then postheating in order to avoid cracking. Maximum hardness on heat-treatment is determined largely by virtue of the alloy content and the high carbon content. The hardening temperature used is about 1475 F. and an oil quench is customarily employed instead of water to avoid cracking. Tempering, to alleviate brittleness introduced through hardening is had at a temperature of about 800 F., the SAE 4645 steel (4600 series with .45 carbon) in the quench-hardened and tempered condition has an ultimate tensile strength of about 200,000 p.s.i., a yield strength of 181,000 p.s.i., an elongation in 2" of 15%, a reduction in area of 49% and a hardness of 420 Brinell. As quenched, the hardness is about Rockwell 055-60.

In addition to the shortcomings of possible cracking, distortion and dimensional changes, the low alloy nickelmolybdenum steels of high carbon content are inclined to scale at the high hardening temperatures required. Moreover, it see-ms that in larger sections the material does not harden uniformly throughout the section. As a result the field of useful application is seriously limited in matters of size, shape and configuration of the work. The known nickel-molybdenum steels do not readily lend themselves to welding without fear of cracking.

One of the objects of my invention, there-fore, is to provide an alloy steel, which steel, while in the annealed condition, readily lends itself to fabrication into a variety of forms and shapes and which, during subsequent hardening in simple one-step ageing operation or in two-step treatment, both under conditions of comparatively low hardening temperatures and of relative short duration, is characterized by minimum scaling and physical distortion and, in the hardened condition is characterized by hard ness and strength, both at room temperatures and at elevated temperatures.

' Considering now the practice of my invention, I find that with a proper combination of the ingredients nickel and molybdenum in a steel of low carbon content I achieve an alloy steel which is subsequently hardenable by heat-treatment from a soluble solution-treated (annealed) condition. This hardening is had by single-step hardening operation of comparatively short duration conducted at relatively low temperatures. Where desired, as for example for maximum ease of working and forming in the solution-treated condition, the hardening is had by a two-step treatment. The resulting hardened steel, whether by single-treatment or by double-treatment, is strong and hard both at room temperatures and at high temperatures. The mechanical properties of my steel compare favorably with known precipitation-hardened steels. And the steel welds much better than the high carbon steels of the 4600 and 4800 series without necessity either for heating prior to welding or for annealing after welding.

The alloy steel of my invention essentially contains about 4% to 30% nickel and about 4% to 20% molybdenum, with remainder substantially all iron. Carbon is included in amounts up to about 0.15%. Manganese also may be present in amounts up to about 4.0%. Phosphorus is present in amounts up to 0.05% maximum, sulphur up to 0.05 maximum, and silicon up to about 3.0%. Where desired, chromium may be added in amounts up to 5.0%. Also, for special purposes, there may be added copper in amounts up to 3.5%. And, where desired, columbium and/ or tantalum is employed in amounts up to 1.0% So, too, there may be added titanium and/or aluminum in amounts up to 1%. In some instances, where extreme hardness is required and some calculated sacrifice in ductility of the resulting alloy steel is permissible, tungsten may be substituted for molybdenum, this up to amounts of about 8% tungsten.

The amounts of nickel and molybdenum employed in my steel and the relation between the same are found to be critical. For if the steel is too high both in nickel and molybdenum, certain practical difliculties are encountered. Where these ingredients are too low, the desired hardening is not had. Similarly, the desired hardening is not had with molybdenum too high and nickel too low. So, too, with nickel too high and molybdenum too low the hardening achieved is wholly inadequate.

In the annealed or solution-treated condition the steel of my invention displays maximum ductility and formability. In the relatively soft and ductile condition thus realized, the steel can readily be worked to desired size and configuration. And the steel products obtained readily lend themselves to hardening by a single-step or double-step treatment conducted at comparatively low temperatures and enduring for relatively short intervals of time. And the resulting age-hardened products are strong and durable. These qualities are retained during prolonged operation at both room temperatures and elevated temperatures.

Within the broad range of composition the single-step treatment steel according to my invention essentially consists of nickel about 4.0% to 12.0%, molybdenum about 4.0% to 12%, carbon up to about 0.15% maximum; manganese up to about 4.0% maximum; silicon up to about 2.0%; and remainder substantially all iron.

And also within the broad composition range set forth above, the double-treatment steel analyzes about to 30% nickel, 6.0% to 20.0% molybdenum, up to 0.05% carbon, up to 3.0% manganese, up to 3.0% silicon, and remainder substantially all iron.

-I preliminarily heat the steel of my invention, both the single-treatment steel and the double-treatment steel, to bring the same into a soft, ductile condition which is suitable for fabricating operations.

The single-treatment steel preferably is annealed at a temperature of about 1500 F. to 2100 F. for a period of time up to 1 hour. The relatively high annealing temperature places the metal in an austenitic and molybdenum-soluble condition. The steel when cooled as by quenching in air, oil or water, is transformed to a substantially martensitic condition, with molybdenum in solution. The steel is usually free of delta-ferrite. Moreover, it is ductile and is substantially free of directionality, together with hardness of about Rockwell C30. Although the duration of the annealing treatment is not too critical I prefer a treatment enduring for approximately one-half an hour. The annealing treatment is conducted in any heat-treating furnace suitable for the purpose. And this treatment usually is effected at the steel mill.

The double-treatment steel of my invention preferably is solution-treated at a temperature of some 1500 F. to 2100 F. for one-half hour or more and then cooled to room temperature. The steel is largely in the austentic condition with minor amounts of delta ferrite. The hardness is well under Rockwell B100. This treatment where employed likewise is usually performed at the mill.

Both the single-treatment steel and the double-treatment steel of my invention, following solution-anneal, is ordinarily shipped to a customer fabricator. And by the customer it is readily formed and fabricated as by punching, bending, stretching, shrinking, or the like, or by drilling, cutting, threading, and the like. It readily lends itself to welding in accordance with the practice of many well-known welding operations. And, where desired, it is readily brazed or soldered. In short, my steel in its solution-treated condition can be readily processed as desired. Particularly is this true of the double-treatment steel for, as recited above, with that steel greater ductility and formability is achieved because of the aus tentic structure and a hardness of significantly lower value.

I harden the single-treatment steel by aging the same at a relatively low temperature for but a short period of time. Ageing ordinarily is conducted at a temperature of approximately 850 F. to 1250 F. for a period of approximately one hour, although this may range up to 24 hours. Following the ageing treatment, I cool the steel, as in air or water. This single-step, low-temperature ageing treatment, of relatively short duration, brings about a precipitation of a molybdenum-rich phase or perhaps a nickel-molybdenum compound. It may be supposed that it is this precipitated phase, rich in molybdenum or the nickel-molybdenum compound which lends the hardness and high physical strength had in the hardened steel.

The short duration of the ageing treatment and the A comparatively low temperatures at which ageing is conducted combine to ensure a minimum of scaling while preventing undesired distortion of the product undergoing treatment.

The double-treatment steel, which as noted above, usually is solution-treated at the higher temperatures, preferably is hardened by a double heat-treatment. This steel in solution-treated condition, and in the form of the desired article, is brought to a temperature of about 12 50 F. to 1450 F. for a time up to 24 hours and then cooled to a temperature of F. to 320 F. for at least 30 minutes. This efiects transformation to a martensitic condition, or a ferritic condition depending upon the carbon content.

Alternatively, where desired, transformation may be had by mechanically cold-Working the solution-treated metal, as in forming and fabrication, until a ferritic condition is developed. In general, however, I prefer to employ the transformation heat-treatment.

Following transformation, either by heat-treatment or cold-working, final hardening of the transformed steel is had by reheating at a temperature of 800 to 1200 F. for a period of time ranging up to 24 hours. The hardness had amounts to Rockwell C38 to C55.

As suggested, following quenching, the steel whether hardened by single-treatment or by double-treatment, is found to have acquired high hardness and high strength. Importantly, and unlike many known precipitation-hardened alloys, the metal retains these properties even following prolonged operation at high temperatures. This I attribute to the stability of a molybdenum-rich phase or the nickel-molybdenum compound present in the hardened steel. The steel has good high temperature properties. This important advantage may be noted upon comparison with many alloys, which in sharp contrast characteristically suffer a loss in these qualities, when operating at elevated temperatures.

Of the single-treatment steel of my invention, one preferred composition analyzes approximately 6% to 8% nickel, 6% to 8% molybdenum, and remainder substan tially all iron. A further preferred composition analyzes approximately 7% to 9% nickel, 8% to 10% molybdenum, and remainder substantially all iron. Another analyzes approximately 9% to 11% nickel, 9% to 11% molybdenum, and remainder substantially all iron. A further analyzes approximately 6% to 8% nickel, 8% to 10% molybdenum, 2% to 4% copper, and remainder substantially all iron. In these steels the incidentals are about .03% carbon, 50% manganese, .010% phosphorus, 010% sulfphur, and about .60% silicon.

Illustrative of the double-treatment steel, a preferred steel analyzes about 18% to 20% nickel, 9% to 10% molybdenum, and remainder substantially all iron. Another analyzes about 13% to 15% nickel, 9% to 11% molybdenum, 1% to 3% chromium, and remainder substantially all iron.

Whithin the broad range of composition, I prepared several steels of single-treatment composition, this as given in the Table I below:

Specimens from the several steels iudentified in Table I were solution-treated at 2000 F. for /2 hour and quenched in oil. Thereafter they Were hardened by heating at 1050 F. and cooled. The hardnesses of these specimens in the solution-treated condition and in the hardened condition are given in Table II below:

TABLE II Room Temperature Hardness of the Steels of Table 1 Heat No. 2,000 I hr. 1,050 F.-1 hr.

oil quench air cool R2434-1 Rockwell C30 Rockwell C49. R2435-1 Rockwell C29 Rockwell C52. R2436-1 Rockwell C29 Rockwell C52.

TABLE III Specific Examples of Double-Treatment Ni-Mo Steel Heat No. 0 Mn P S Si Cr Ni M0 The example of Table III was annealed at 1950" F. for /2 hour and water-quenched. It was then reheated at 1450 F. for 1% hours and water-quenched in order to transform the metal to martensite. Following transformation it was reheated at 1050 F. for 1 hour and aircooled. The hardness had in the annealed condition, the transformed condition and the final hardened condition is given in Table IV below:

TABLE IV Room Temperature Hardness of the Steel 0 Table III in the Annealed, Transformed and Hardened Conditions Heat No.

Rockwell C32 Rockwell C47.

R2579. Rockwell B84-..

In the annealed condition note that the hardness amounts to Rockwell B84, while in the fully hardened condition it is Rockwell C47. By comparing these figures with those for the single-treatment steel (Table II, annealed hardness of Rockwell C30 and final hardness of Rockwell C49), it will be seen that lower annealed hardness and greater ductility is had with the doubletreatment steel, this in combination with a hardness in the final age-hardened condition which is about the same as the single-treatment examples.

Thus it will be seen that I provide a steel and method of heat-treating the same, in which the many objects and advantages hereinbefore set forth are successfully achieved. My steel displays superior mechanical properties, particularly strength and hardness; it is well suited to duty at room temperatures as well as at temperatures ranging up to about 1100 F. And this combination of properties is had with only the two essential alloying ingredients nickel and molybdenum, all with simple melting procedure and at minimum expense.

The steels of my invention are suited to the production of castings as well as the production of wrought articles of intrictae detail and form. The metal in the solutiontreated or annealed condition, whether cast or wrought, is readily machined, welded and brazed. In wrought condition, as in the form of plate, sheet, strip, bars, rods and wire, the metal lends itself to a variety of forming 6 operations such as bending, pressing, stretching and the like.

It is apparent from the foregoing that I provide in my invention a nickel-molybdenum alloy steel, together with a single-treatment method of hardening the same, as well as a double-treatment, in the course of both of which I achieve the various objects pointed out above along with many thoroughly practical advantages. Inasmuch as many embodiments may be made of my invention as disclosed above, and many changes and modifications made of the disclosed embodiments, it will be understood that the foregoing disclosure is to be considered as illustrative, and not as a limitation.

I claim as my invention:

1. Nickel-molybdenum alloy steel plate, sheet, strip, bars, wire and like products solution-treated at 1500" to 2100 F. to a hardness well under Rockwell B100, said products essentially consisting of about 10% to 30% nickel, 6% to 20% molybdenum, and remainder essentially iron, said products being characterized by a molyb denum-soluble and nickel-soluble condition, as well as ductility and workability, and by being precipitationhardenable to Rockwell C38-55 together with strength and durability both at room temperatures and elevated temperatures, when transformed by heating at 1250 to 1450 F. and cooling to about to 320 F. and then reheated at 800 to 1200 F 2. Nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and like products solution-treated at 1500 to 2100 F. to a hardness well under Rockwell B100, said products essentially consisting of about 10% to 30% nickel, 6% to 20% molybdenum, and remainder essentially iron, and being precipitation-hardenable to Rockwell C38-55, with resulting strength and durability both at room temperatures and elevated temperatures, when transformed by heating at l250 to 1450 F. and cooling to about 125 to 320 F., and then hardened by reheating at 800 to 1200 F.

3. Nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and like products solution-treated at 1500 to 2100 F. to a hardness well under Rockwell B100, said products essentially consisting of about 18% to 20% nickel 9% to 11% molybdenum, and remainder essentially iron, and being precipitation-hardenable to Rockwell C38-55, with resulting strength and durability both at room temperatures and elevated temperatures, when transformed by heating at 1250 to 1450 F. and cooling to about 125 to 320 F., and then hardened by reheating at 800 to 1200 F.

4. Nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and like products solution-treated at 1500 to 2100 F. to a hardness well under Rockwell B100, said products essentially consisting of about 13% to 15% nickel, 9% to 11% molybdenum, 1% to 3% chromium, and remainder essentially iron and being precipitationhardenable to Rockwell C38-55, with resulting strength and durability both at room temperatures and elevated temperatures, when transformed by heating at 1250 to 1450 F. and cooling to about 125 to 320 F., and then hardened by reheating at 800 to 1200 F.

5. Nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and like products solution-treated at 1500 to 2100 F. to a hardness of about Rockwell C30, said products essentially consisting of about 4% to 12% nickel, 4% to 12% molybdenum, and remainder essentially iron, and being precipitation-hardenable to Rockwell C38-55, with resulting strength and durability both at room temperatures and elevated temperatures, by reheating at 850 to 1250 F.

6. Nickel-molybdenum allow steel sheet, wire, bar, strip, plate and like products solution-treated at 1500 to 2100 F. to a hardness of about Rockwell C30, said products essentially consisting of about 6% to 8% nickel, 8% to 10% molybdenum, 2% to 4% copper, and remainder essentially iron, and being precipitation-hardenable to Rockwell (138-55, with resulting strength and durability both at room temperatures and elevated temperatures, by reheating at 850 to 1250 F.

7. In the production of precipitation-hardened nickelmolybdenum alloy steel articles having a hardness of about Rockwell C3 855, the art which comprises providing nickel-molybdenum alloy steel sheet, Wire, bar, strip, plate and the like in which there are correlated about 10% to 30% nickel, 6% to 20% molybdenum, and remainder essentially iron annealed at 1500 F. to 2100 F. to a hardness well under Rockwell B100; heating the same at l250 to 1450 F. and cooling to about 125 to 320 F. to effect transformation; and then reheating at 800 to 1200 F. to effect precipitation-hardening or" the transformed steel.

8. In the production of precipitation-hardened nickelmolybdenum alloy steel articles having a hardness of about Rockwell C3 8-55, the art which comprises providin nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and the like in which there are correlated about 10% to 30% nickel, 6% to 20% molybdenum, and remainder essentially iron annealed at 1500 to 2100 F. and cooled to a hardness well under Rockwell B100; cold-working the same to effect transformation; and heating at 800 to 1200 F. to effect precipitation-hardening of the transformed steel.

9. In the production of precipitation-hardened nickelmolybdenurn alloy steel articles having a hardness of about Rockwell C3 855, the art which comprises providing nickel-molybdenum alloy steel sheet, Wire, bar, strip,

plate and the like in which there are correlated about 4% to 12% nickel, 4% to 12% molybdenum, and remainder essentially iron annealed at 1500 to 2l00 F. and cooled to a hardness of about Rockwell C30; and heating the same at about 850 to 1250 F. to effect precipitationhardening.

10. Precipitation-hardened articles of nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and the like, with hardness of about Rockwell C3855, consisting of approximately 10% to 30% nickel, 6% to 20% molybdenum, and remainder essentially iron transformed and then precipitation-hardened by heat-treatment.

11. Precipitation-hardened articles of nickel-molybdenum alloy steel sheet, wire, bar, strip, plate and the like, with hardness of about Rockwell C3855, consisting of approximately 4% to 12% nickel, 4% to 12% molybdenum, and remainder essentially iron, solutiontreated and precipitation-hardened by heat-treatment.

References Cited in the file of this patent UNITED STATES PATENTS 2,185,996 Hatfield Ian. 9, 1940 2,614,921 Tanczyn Oct. 21, 1952 2,694,626 Tanczyn Nov. 16, 1954 OTHER REFERENCES Kester: Archiv. fiir Das Eisenhuttenwescn, Heft 4, Gctober 1934, pp. l6917l (Das System Bison-Nickell tolybdan). 

1. NICKEL-MOLYBDENUM ALLOY STEEL PLATE, SHEET, STRIP, BARS, WIRE AND LIKE PRODUCTS SOLUTION-TREATED AT 1500* TO 2100*F. TO A HARDNESS WELL UNDER ROCKWELL B100, SAID PRODUCTS ESSENTIALLY CONSISTING OF ABOUT 10% TO 30% NICKEL, 6% TO 20% MOLYBDENUM, AND REMAINDER ESSENTIALLY IRON, SAID PRODUCTS BEING CHARACTERIZED BY A MOLYBDENUM-SOLUBLE AND NICKEL-SOLUBLE CONDITION, AS WELL AS DUCTILITY AND WORKABILITY, AND BY BEING PRECIPITATIONHARDENABLE TO ROCKWELL C38-55 TOGETHER WITH STRENGTH AND DURABILITY BOTH AT ROOM TEMPERATURES AND ELEVATED TEMPERATURES, WHEN TRANSFORMED BY HEATING AT 1250* TO 1450*F. AND COOLING TO ABOUT 125* TO -320*F. AND THEN REHEATED AT 800* TO 1200*F. 